Method and apparatus for a variable bandwidth multi-protocol X-DSL transceiver

ABSTRACT

A transceiver for communicating a multi-tone modulated communication channel on a subscriber line. The transceiver includes: a digital signal processor (DSP) with a Fourier transform module and an analog front end (AFE). The DSP determines an available range of frequencies on the subscriber line and expands or contracts the tone spacing of each of a fixed number “N” of tones accordingly by decreasing or increasing the processing interval associated with the Fourier transform of each tone set. The AFE performs digital-to-analog conversion of the multi-tone modulated communication channel at rates compatible with the processing interval of the Fourier transform module; whereby the range of frequencies spanned by the modulated tones on the subscriber line conforms to the available of frequencies on the subscriber line.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of prior filed co-pendingProvisional Application No. 60/197,713 filed on Apr. 18, 2000 entitled“Programmable and Variable Bandwidth DMT Engine” which is incorporatedherein by reference in their entirety as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of Invention

The field of the present invention relates in general to modems and moreparticularly to a method and apparatus for a variable bandwidth X-DSLmodem.

2. Description of the Related Art

North American Integrated Service Digital Network (ISDN) Standard,defined by the American National Standard Institute (ANSI), regulatesthe protocol of information transmissions over telephone lines. Inparticular, the ISDN standard regulates the rate at which informationcan be transmitted and in what format. ISDN allows full duplex digitaltransmission of two 64 kilo bit per second data channels. These datarates may easily be achieved over the trunk lines, which connect thetelephone companies' central offices. The problem lies in passing thesesignals across the subscriber line between the central office and thebusiness or residential user. These lines were originally constructed tohandle voice traffic in the narrow band between 300 Hz to 3000 Hz atbandwidths equivalent to several kilo baud.

Digital Subscriber Lines (DSL) technology and improvements thereonincluding: G.Lite, ADSL, VDSL, HDSL all of which are broadly identifiedas X-DSL have been developed to increase the effective bandwidth ofexisting subscriber line connections, without requiring the installationof new fiber optic cable. An X-DSL modem operates at frequencies higherthan the voice band frequencies, thus an X-DSL modem may operatesimultaneously with a voice band modem or a telephone conversation.Currently there are over ten discrete X-DSL standards, including:G.Lite, ADSL, VDSL, HDSL2, SHDSL, and other DSLs all of which arebroadly identified as X-DSL.

Currently there are over ten discrete XDSL standards, including: G.Lite,ADSL, VDSL, SDSL, MDSL, RADSL, HDSL, etc. Within each standard there areat least two possible line codes, or modulation protocols, discretemulti-tone (DMT) and carrierless AM/PM (CAP). A typical DMT systemutilizes a transmitter inverse discrete Fourier transform (IDFT) and areceiver discrete Fourier transform (DFT). The following patents arerelated to DMT modems: U.S. Pat. No. 5,400,322 relates to bit allocationin the multi-carrier channels; U.S. Pat. No. 5,479,447 relates tobandwidth optimization; U.S. Pat. No. 5,317,596 relates to echocancellation; and U.S. Pat. No. 5,285,474 relates to equalizers. Thefollowing patents are related to CAP modems: U.S. Pat. No. 4,944,492relates to multidimensional pass band transmission; U.S. Pat. No.4,682,358 relates to echo cancellation; and U.S. Pat. No. 5,052,000relates to equalizers. Each of these patents is incorporated byreference as if fully set forth herein.

XDSL modems are typically installed in pairs, with one of the modemsinstalled in a home or business and the other in the telephone companiescentral office (CO) switching office servicing that home. This providesa direct dedicated connection to the home or office from a line card atthe central office on which the modem is implemented through thesubscriber line or local loop.

Each installation represents a sizeable expense in hardware and servicelabor to provision the central office. The expense may not always beamortized over a sufficient period of time due the relentlessintroduction of new and faster xDSL standards each of which pushes theperformance boundaries of the subscriber line in the direction ofincreasing bandwidth and signal integrity. As each new standardinvolves, line cards must typically be replaced to upgrade the service.Not all subscriber lines qualify for the higher bandwidths offered bythe evolving X-DSL protocols. Subscriber line length, i.e. the distancefrom the home or business to the central office is one of the primaryfactors determining the ability of a subscriber line to support higherdata rates. Evolving standards like VDSL call for data rates from 3.75Mega Bits per second (Mbps) up to 13 Mbps and higher. Typically only asmall percentage of the installed subscriber lines with loop distancesless than 2000 feet from the central office will qualify for the upperdata rates, the higher bandwidths. The hardware and processingcapability needed to deliver high bandwidths to these short loops isexpensive, and can not be utilized on the longer subscriber loops.

What is needed is a less rigid signal processing architecture thatsupports scalability of CO resources, and allows a more flexiblehardware response to the evolving XDSL standards and the problemsassociated with providing hardware to handle each new standard.

SUMMARY OF THE INVENTION

An apparatus and method is disclosed for a variable bandwidth X-DSLmodem. The modem implements a discrete multi-tone (DMT) line code withvarying tone spacing depending on the bandwidth availability on selectedsubscriber lines. For short subscriber loops that qualify for high datarates the spacing between tones in a tone set is expanded to support thehigher data rates. For longer subscriber loops that do not qualify forhigh data rates the same tone set at a more compact tone spacing isutilized to modulate the subscriber line data. The benefit to thisapproach is that the Fourier engine portion of the modem needed tomodulate and demodulate the channels uses the same number of tones tosupport both high and low data rates. This reduces the overall cost andcomplexity of the modem and avoids a significant increase in system costfor limited symmetrical applications for which a larger discrete Fouriertransform DFT and inverse discrete Fourier transform IDFT wouldotherwise be required. When utilized in a multi-channel environment thevariable bandwidth approach of the current invention allows pipeliningof multiple channels across corresponding subscriber lines at a mix ofdata rates.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent to those skilled in the art from the followingdetailed description in conjunction with the appended drawings in which:

FIG. 1 shows a communication system with a pair of multi-protocolmulti-channel modem line cards coupled to one another by a binder ofsubscriber lines between a public switched telephone network (PSTN)central office (CO) and a remote site.

FIG. 2 is a detailed hardware block diagram of one of the logical modemline cards shown in FIG. 1.

FIGS. 3 A-B are signal diagrams showing a common tone set with defaulttone spacing and expanded tone spacing for handling variable bandwidthVDSL communications over long and short subscriber loops respectively.

FIGS. 4 A-B are hardware block diagrams showing a detailed logical viewof the logical modem shown in FIG. 2 during the transmission andreception of a default tone spacing and an expanded tone spacingrespectively.

FIG. 5 shows the various DMT frame sizes associated with the logicalmodem shown in FIG. 2, including a new frame structure suitable for theexpanded tone spacing associated with short subscriber loops whichqualify for elevated data rates.

FIG. 6 A shows possible session allocations for the multi-channellogical modem shown in FIG. 2.

FIGS. 6B-D show the possible scheduling in the DSP for each of thechannels of each of the three sessions shown in FIG. 6A.

FIG. 7 is a process flow diagram of the setup phase of operation of themodem shown in FIG. 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An apparatus and method is disclosed for a variable bandwidth X-DSLmodem. The modem implements a discrete multi-tone (DMT) line code withvarying tone spacing depending on the bandwidth availability on selectedsubscriber lines. For short subscriber loops that qualify for high datarates the spacing between tones in a tone set is expanded to support thehigher data rates. For longer subscriber loops that do not quality forhigh data rates the same tone set at a more compact tone spacing isutilized to modulate the subscriber line data. The benefit to thisapproach is that the Fourier engine portion of the modem needed tomodulate and demodulate the channels uses the same number of tones tosupport both high and low data rates. This reduces the overall cost andcomplexity of the modem and avoids a significant increase in system costfor limited symmetrical applications for which a larger discrete Fouriertransform DFT and inverse discrete Fourier transform IDFT wouldotherwise be required. When utilized in a multi-channel environment thevariable bandwidth approach of the current invention allows pipeliningof multiple channels across corresponding subscriber lines at a mix ofdata rates. The apparatus may be applied with equal advantage in wiredand wireless media.

In an embodiment of the invention the X-DSL transceiver implements theDMT protocol and includes a digital signal processor (DSP) and an analogfront end (AFE) coupled to at least one subscriber line. A DFT/IDFTengine is implemented in the DSP with a DFT portion to convert digitizedtone sets on a receive path for each channel to digitized symbols and anIDFT portion to convert for the digitized symbols on the transmit pathto digitized tone sets. The DFT/IDFT engine provides variable tonespacing for the at least one channel. A variable rate interpolatorcouples to the IDFT portion of the DFT/IDFT engine and sets the samplingrate at the output of the IDFT to match the sampling rate of a digitalinput to the digital-to-analog (DAC) portion of the AFE. A variable ratedecimator couples to the digital output of the AFE and sets the samplingrate at the digital output of the AFE to match the sampling rate at theinput of the DFT portion of the DFT/IDFT engine. In an embodiment of theinvention a scheduler couples to the DFT/IDFT engine for schedulingchannels to be processed by the DFT/IDFT engine during each processinginterval. An initialization procedure is used to determine appropriatetone spacing to be used for the channel based on the loop qualificationsof the channel.

FIG. 1 shows a communication system with a pair of multi-protocolmulti-channel modem line cards coupled to one another by a binder ofsubscriber lines between a public switched telephone network (PSTN)central office (CO) and a remote site. The system includes a CO 100 anda remote line card 156 positioned at a remote terminal 150. The CO andremote line card are coupled to one another via a subscriber line binder170 which includes individual subscriber lines 172,174, 176, 178.

Each of the subscriber line connections terminates on the CO end, in theframe room 102 of the CO. From this room connections are made for eachsubscriber line via splitters and hybrids to both a DSLAM 104 and to thevoice band racks 106. The splitter shunts voice band communications todedicated line cards, e.g. line card 112 or to a voice band modem pool(not shown). The splitter shunts higher frequency X-DSL communicationson the subscriber line to a selected line card, e.g. line card 116,within DSLAM 104. The line cards are universal, meaning they can handleany current or evolving standard of X-DSL and may be upgraded on the flyto handle new standards.

Voice band call set up is controlled by a Telco switch matrix 114 suchas SS7. This makes point-to-point connections to other subscribers forvoice band communications across the public switched telephone network132. The X-DSL communications may be processed by a universal line cardsuch as line card 116. That line card includes a plurality of AFE's118-120 each capable of supporting a plurality of subscriber lines. TheAFEs are coupled via a packet based bus 122 to the DSP 124. Fordownstream communications from the CO to the remote site, the DSPmodulates the data for each communication channel, the AFE transformsthe digital symbol packets assembled by the DSP and converts them to ananalog signal which is output on the subscriber line associated with therespective channel. For upstream communications from the remote site tothe CO each received channel is converted within the corresponding AFEto a digitized data sample which is sent to the DSP for demodulation.The DSP is capable of multi-protocol support for all subscriber lines towhich the AFE's are coupled. Communications between AFE's and DSP(s) maybe packet based, in which embodiment of the invention a distributedarchitecture such as will be set forth in the following FIG. 2 may beimplemented. In FIG. 1 the line card 116 is coupled to a back-plane bus128 which may be capable of offloading and transporting low latencyX-DSL traffic between other DSPs for load balancing. The back-plane busof the DSLAM also couples each line card to the Internet 130 via server108. Each of the DSLAM line cards operates under the control of a DSLAMcontroller 110 which handles global provisioning, e.g. allocation ofsubscriber lines to AFE and DSP resources. The various components on theline card form a plurality of logical modems each handling upstream anddownstream communications across corresponding subscriber lines. When anX-DSL communication is established on a subscriber line, a specificchannel identifier is allocated to that communication. That identifieris used in the above mentioned packet based embodiment to track eachpacket as it moves in an upstream or downstream direction between theAFE and DSP. In an alternate embodiment of the invention discrete modemswould each couple to an associated one of the subscriber lines ratherthan the logical modem shown.

At the remote site a similar line card architecture is shown for linecard 156 which forms a plurality of logical modems connected tocorresponding ones of subscriber lines 172, 174, 176, 178. That linecard includes AFEs 158, a packet bus 160 and a DSP 162. In an alternateembodiment of the invention the termination at the remote site 150 wouldbe a set of discrete modems each coupled to an associated one of thesubscriber lines rather than the logical modem shown. These modules, AFEand DSP, may be found on a single universal line card, such as line card156 in FIG. 2. They may alternately be displaced from one another onseparate line cards linked by a DSP bus. In still another embodimentthey may be found displaced from one another across an ATM network.There may be multiple DSP chipsets on a line card. In an embodiment ofthe invention the DSP and AFE chipsets may include structures set forthin the figure for handling of multiple line codes and multiple channels.

FIG. 2 is a detailed hardware block diagram of one of the logical modemline cards shown in FIG. 1. In this embodiment of the invention a packetbased multi-channel transmission architecture is utilized to implementcommunications across the logical modem. In this architecture a DSP 124handles processing for a number of channels of upstream and downstreamsubscriber line communications via a number of AFE's. Each AFE in turnaccepts packets associated with a plurality of subscriber lines to whicheach AFE is coupled. Packet based processing of raw data both between aDSP and AFE as well as within each DSP and AFE is implemented. Packetprocessing between DSP and AFE modules involves transfer of bus packets294 each with a header portion 296 and data portion 298. The headercontains information correlating the data with a specific channel anddirection, e.g. upstream or downstream, of communication. The dataportion contains for upstream traffic digitized samples of the receiveddata for each channel and for downstream packets digitized symbols forthe data to be transmitted on each channel. Packet processing within aDSP may involve device packets 286. The device packets may include aheader 288, a control portion 290 and a data portion 292. The headerserves to identify the specific channel and direction. The header maycontain control information for the channel to be processed. The controlportion 290 may also contain control portions for each specificcomponent along the transmit or receive path to coordinate theprocessing of the packets. Within the AFE the digitized data generatedfor the received (upstream data) will be packetized and transmitted tothe DSP. For downstream data, the AFE will receive in each packet fromthe DSP the digitized symbols for each channel which will be modulatedin the AFE and transmitted over the corresponding subscriber line.

These modules, AFE and DSP, may be found on a single universal linecard, such as line card 116 in FIG. 1. They may alternately be displacedfrom one another on separate line cards linked by a DSP bus. In stillanother embodiment they may be found displaced across an ATM network.

DSP line card 116 includes one or more DSP's. In an embodiment of theinvention each may include structures set forth in the figure forhandling of multiple line codes and multiple channels. The line cardincludes, a DSP medium access control (MAC) 200 which handles packettransfers to and from the DSP bus 122. The MAC couples with a packetassembler/disassembler (PAD) 202. For received DSP bus packets, the PADhandles removal of the DSP bus packet header 296 and insertion of thedevice header 288 and control header 290 which is part of the devicepacket 286. The content of these headers is generated by the coreprocessor 212 using statistics gathered by the de-framer 222. Thesestatistics may include gain tables, or embedded operations channelcommunications from the subscriber side. The PAD embeds the requiredcommands generated by the core processor in the header or controlportions of the device packet header. Upstream device packets (Receivepackets) pass into a first-in-first-out FIFO buffer 208 which iscontrolled by FIFO controller 206. These packets correspond withmultiple protocols and multiple channels. Each is labeled accordingly.The controller 206 operates as a scheduler, handling the interleaving(See FIGS. 6A-D) and delivery of each packet to the discrete Fouriertransform (DFT) engine 204 which is the next process on the upstream(receive) path (See FIGS. 6A-D).

The receive processing engine 204 implements a discrete Fouriertransform (DFT) on the raw digital data transforming a complete tone setfor one channel from the time to the frequency domain. The outputs arecomplex number coefficients, with each complex number containing thephase and amplitude for a corresponding tone of the tone set. The DFTengine 204 fetches packets and processes the data in them in a mannerappropriate for the protocol, channel and command instructions, if any,indicated by the packet header. The DFT engine is reconfigurable tosupport varying numbers of tones depending on the protocol implementedfor a specific channel. Each tone set corresponds to a symbol. The DFTengine processes successive tone sets for each channel periodically. Theprocessing interval of each DFT engine is programmable. The processinginterval for selected ones of the channels with loop qualification forhigher data rates, i.e. “short loops”, is conducted at an integerfraction of the processing interval of 250 micro seconds called for bythe existing X-DSL DMT standards. The processing of successive tone setsof other channels with loop qualification which do not support higherdata rates is conducted at the periodicity called for by the existingX-DSL standard. Currently the X-DSL standard calls for processingperiodicity of 250 micro seconds for successive tone sets of eachchannel, which corresponds to a tone spacing of 4.3125 kHz (See FIG.3A). Utilizing the current invention some of these channels with loopqualification for higher data rates may be periodically processed atintervals which are an integer fraction, e.g. ½ of the standard. Wherethe processing periodicity for successive tone sets is ½ the standard or125 micro seconds the tone spacing expands to 8.625 kHz. (See FIG. 3B).This allows the same DFT engine with the same sample size, the samenumber of tones, to provide up to twice the upstream bandwidth. Otherchannels which do not qualify for higher data rates are processed at the250 micro second interval called for by the standard.

The higher throughput of the DFT engine may be achieved with a singlehigh speed DFT engine or a set of discrete DFT engines operating inparallel on the successive tone sets of each channel.

The output of the DFT engine for each channel are successive sets ofcomplex coefficients for each tone in the set. These contain phase andamplitude information for the information modulated on each tone andcollectively make up a symbol. Next, the coefficients for each tone setare passed in a packet for each channel to the de-framer and decoder222. In this module each symbol is decoded, Reed Solomon or other errorcorrection is implemented and de framed. For a channel with expandedtone sets each frame includes two DMT symbols (See FIG. 5). This modulereads the next device packet and processes the data in it in accordancewith the instructions or parameters in its header.

The de-framed data is passed to the final FIFO buffer 226 which iscontrolled by controller 204. That data is then passed to the ATM pad228 for wrapping with an ATM header and removal of the device header.The ATM MAC 230 then places the data with an ATM packet on the ATMnetwork 130 (see FIG. 1).

Control of the receive modules, e.g. DMT engine 204 and de-framerdecoder 222 as well as sub modules thereof is implemented as follows.The core processor 210 has DMA access to the FIFO buffer 226 from whichit gathers statistical information on each channel including gaintables, or gain table change requests from the subscriber as well asinstructions in the embedded operations portion of the channel. Thosetables 214 are stored by the core processor in memory 212. When a changein gain table for a particular channel is called for the core processorsends instructions regarding the change in the header of the devicepacket for that channel via PAD 202 and writes the new gain table to amemory which can be accessed by the appropriate module, i.e. DMT module204 or the appropriate sub module thereof as a packet corresponding tothat channel is received by the module. This technique of in bandsignaling with packet headers allows independent scheduling of actionson a channel by channel basis in a manner which does not require thedirect control of the core processor. Instead each module in thetransmit path can execute independently of the other at the appropriatetime whatever actions are required of it as dictated by the informationin the device header which it reads and executes.

This device architecture allows the DSP transmit and receive paths to befabricated as independent modules or sub modules which respond to packetheader control information for processing of successive packets withdifferent X-DSL protocols, e.g. a packet with ADSL sample data followedby a packet with VDSL sampled data. Within the DMT Rx engine 204 forexample, there may be sub modules with independent processing capabilitysuch as: a time domain equalizer, a cyclic prefix remover, a DFT, a gainscalar, a trellis decoder and a tone reorderer, as well as filters, awindowers . . . etc. Each of these sub modules has its counterpart onthe DMT Tx engine 220 in the transmit path. Each of these mayindependently respond to successive device headers to change parametersbetween successive packets. For example as successive packets fromchannels implementing G.Lite, ADSL and VDSL pass through the DMT Txengine the number of tones will vary from 128 for G.lite, to 256 forADSL, to 2048 for VDSL. In accordance with the current invention VDSLmay be implemented with 2048 tones with a processing periodicity ofeither 250 micro seconds as called for by the standard of somefractional part thereof. In alternate embodiments of the invention tonespacing expansion may be implemented on any protocol or tone set size.The framer and de-framer will use protocol specific informationassociated with each of these channels to look for different frame andsuper frame boundaries, including the dual symbol frame set forth in thefollowing FIG. 5 for handling tone sets with expanded tone spacing.

On the downstream side, i.e. Transmit, the same architecture applies.ATM data which is unwrapped by PAD 228 is re-wrapped with a deviceheader the contents of which are again dictated by the core processor210. That processor embeds control information related to each channelin the packets corresponding to that channel. The device packets arethen passed to the FIFO buffer 232 which is controlled by controller234. The controller handles the sequencing and delivery of each packetto the Framer and Reed-Solomon (RS) encoder 236 and or sub modulesthereof then processes these packets according to the informationcontained in their header or control portions of each device packet. TheFramer then updates the device packet header and writes the resultantdevice packet to the IDFT transmit module 220. This module accepts thedata and processes it for transmission. Transmission processing mayinclude: tone ordering, trellis encoding, gain scaling, an IDFT, andcyclic prefix modules each with independent ability to read and respondto device headers. Where a specific channel loop qualifies for expandedtone spacing and concomitant higher data rates the IDFT implements theserates by periodically generating a tone set in an interval which is aninteger fraction of that called for by the standard. Where theprocessing periodicity for successive coefficient sets is ½ the standardor 125 micro seconds the tone spacing expands to 8.625 kHz. (See FIG.3B). This allows the same IDFT engine with the same sample size, thesame number of tones, to provide up to twice the upstream bandwidth.Other channels which do not qualify for higher data rates have theircoefficient sets transformed into tone sets at the 250 micro secondinterval called for by the standard. The DFT engine 204 and the IDFTengine 220 collectively form an IDFT/DFT engine which may be implementedin hardware, firmware or software. The IDFT/DFT engine may beimplemented in discrete portions for the transmit and receive paths oras a single engine shared between the transmit and receive paths.

From the DMT Tx engine 220 each updated device packet with a digitizedsymbol(s) for a corresponding channel is placed in the FIFO buffer 216under the control of controller 218. From this buffer the device packetis sent to PAD 202 where the device header is removed. The DSP PADplaces the DSP packet 294 with an appropriate header onto the DSP bus122 for transmission to the appropriate AFE and the appropriate channeland subscriber line within the AFE.

Because the data flow in the AFE allows a more linear treatment of eachchannel of information an out of band control process is utilized withinthe AFE. In contrast to the DSP device packets which are used tocoordinate various independent modules within the DSP the AFEaccomplishes channel and protocol changeovers with a slightly differentcontrol method.

A packet on the bus 122 directed to AFE 120 is detected by AFE MAC 240on the basis of information contained in the packet header. The packetis passed to PAD 242 which removes the header 296 and sends it to thecore processor 244. The packet's header information including channel IDis stored in the core processor's memory 248. The information iscontained in a table 246. The raw data 298 is passed to a FIFO buffer252 under the control of controller 250. Each channel has a memorymapped location in that buffer.

On the transmit path, the interpolator 254 reads a fixed amount of datafrom each channel location in the FIFO buffer. The amount of data readvaries for each channel depending on the bandwidth of the channel. Theamount of data read during each bus interval is governed by entries inthe control table for each channel which is established during channelsetup and is stored in memory 248. The interpolator up samples the dataand low pass filters it to reduce the noise introduced by the DSP.Implementing interpolation in the AFE as opposed to the DSP has theadvantage of lowering the bandwidth requirements of the DSP bus 122. Theamount of interpolation will vary between channels with expanded tonespacing and channels with “standard” tone spacing of 4.3125 kHz. In anembodiment of the invention in which all analog stages run at a commonsample rate the interpolation factor for a channel with expanded tonespacing will be less than that to which a channel with normal tonespacing will be subject. This allows all channels irrespective of tonespacing to be handled on analog stages with a common clock rate. In analternate embodiment of the invention clock rates for the subsequentanalog stage would vary depending on the tone spacing of thecorresponding channel.

From the interpolator data is passed to the correspondingdigital-to-analog converter (DACs) 256,258. The analog outputs of eachDAC are introduced to the corresponding amplification and filteringstage i.e. amplifier 260 together with filter 264 and amplifier 262together with filter 266, from which they are coupled to correspondingsubscriber lines. Each of the transmit modules 254-266 may be coupled tothe control processor 244. The parameters for each of the transmitmodules, i.e. filter coefficients, amplifier gain etc. are controlled bythe core processor using control parameters stored during session setup. For example, where successive packets carry packets with G.Lite,ADSL, and VDSL protocols the interpolation factors, sample rate, filterparameters for and the gain of the analog amplifiers will vary betweenthe packets associated with each channel. This “on the fly”configurability allows a single transmit or receive pipeline to be usedfor multiple concurrent protocols.

On the upstream path, the receive path, individual subscriber linescouple to individual line amplifiers, e.g. 270-272, through splitter andhybrids (not shown). Each channel is passed to dedicated universal X-DSLfilters 274-276. These filters, one per subscriber line, are configuredto handle substantial attenuation of cross-talk from the transmit pathfor whichever of the X-DSL protocols may be implemented on thesubscriber line. Next the ADC modules 278-280 convert the correspondingfiltered analog signals to digital signals. Each channel may be subjectto further digital filtering and decimation 278.

The amount of decimation will vary between channels with expanded tonespacing and channels with “standard” tone spacing. In an embodiment ofthe invention in which all analog stages run at a common sample rate thedecimation factor for a channel with expanded tone spacing will be lessthan that to which a channel with normal tone spacing will be subject.This allows all channels irrespective of tone spacing to be handled onanalog stages with a common clock rate. In an alternate embodiment ofthe invention clock rates for the subsequent analog stage would varydepending on the tone spacing of the corresponding channel.

As discussed above in connection with the transmit path, each of thesecomponents is configured on the fly for each new packet depending on theprotocol associated with it. Each channel of data is then placed in amemory mapped location of FIFO memory 286 under the control ofcontroller 284. Scheduled amounts of this data are moved to PAD 242during each bus interval. The PAD wraps the raw data in a DSP headerwith channel ID and other information which allows the receiving DSP toproperly process it.

FIGS. 3 A-B are signal diagrams showing a common set of DMT tones with astandard tone spacing 300 of 4.3125 kHz (See FIG. 3A) and an expandedtone spacing of 8.625 kHz (See FIG. 3B). The variation in tone spacingis achieved by varying the processing periodicity for successivesymbol/tone sets from the X-DSL standard of 250 microseconds per symbolper tone set to 125 microseconds per symbol per tone set for eachchannel. Each tone set for the communication channel 300 shown in FIG.3A is periodically processed by the DFT/IDFT at an interval of 250microseconds. Each tone set for the communication channel 310 shown inFIG. 3B is periodically processed by the DFT/IDFT at an interval of 125microseconds. The number of tones in FIG. 3A is the same as the numberin FIG. 3B, thus the same sample size DFT/IDFT may be used to processboth channels. In FIG. 3A there are two downstream communicationbandwidths 330 and 334 and one upstream communication bandwidth 332. Thefirst downstream communication channel is bounded by lower tone 302 andan upper tone 304 each with a corresponding tone number. In FIG. 3B thesame tone set has been expanded to provide coverage for the same twodownstream communication bandwidths 330 and 334, the same one upstreamcommunication bandwidth 332 and a new upstream bandwidth 336. Thislatter frequency range is accessible through the same DFT/IDFT samplesize due to the decreased period of the sample/tone set processingeffected by the DFT/IDFT engines for channel 310. Both the number oftones and the boundary tone numbers for each upstream and downstreamcommunication bandwidth have changed. The first downstream band 330 isbounded by tones referenced as 312 and 314. The tone orderer which is asub module (not shown) of the framer and RS encoder 236 (See FIG. 2) onthe transmit path and the tone de-orderer (not shown) which is a submodule of the de-framer and RS decoder 222 on the receive path vary thetone numbering accordingly during the processing of each of the packetscorresponding with channels 300 and 310.

FIGS. 4 A-B are hardware block diagrams showing a detailed logical viewof the logical modem shown in FIG. 2 during the transmission andreception of the channels 300 and 310 shown in FIGS. 3A-B respectively.In FIG. 4A the signal processing for channel 300 with a standard tonespacing is shown. In FIG. 4B the signal processing for channel 310 withan expanded tone spacing is shown. The X-DSL transceiver of the currentinvention may be implemented either as a multi-channel device capable ofhandling concurrently channels with both standard and expanded tonesets. In an alternate embodiment of the invention the X-DSL transceivermay be implemented as a single channel device capable of switching fromstandard tone spacing to expanded tone spacing depending on the loopqualification of the subscriber line to which it is coupled.

In FIG. 4A the processing of the first channel 300 with default tonespacing is shown. After framing, tone ordering, RS encoding and gainscaling in module 236 the coefficients for each symbol are converted inthe IDFT module 220 from the frequency to the time domain. Thisconversion occurs at a sampling frequency of Fs=X. The samplingfrequency equals the number of tones in a tone set plus a specifiedprefix or suffix multiplied by the one over the standard processinginterval of 250 microseconds. Next the digital time domain data for eachsymbol set is passed to the variable rate interpolator 254 which in theembodiment shown is part of the AFE, though such need not be the case.

In the interpolator 254 there are logically two interpolation stages 412and 414. In stage 414 the an interpolation by a factor “n” occursfollowed by a further interpolation by a factor, e.g. 2 in stage 412.The interpolation that occurs in stage 412 is common to both the signalprocessing for channel 300 with standard tone spacing and channel 310with expanded tone spacing. The interpolation in stage 414 is necessarywhen the DAC 260 runs at a constant rate for the signal processing ofeither channel 300 or 310. The channel 300 is subject to an additionalinterpolation in stage 414. The interpolation factor in this stage isthe inverse of the integer fraction by which the processing periodicityof channel 310 correlates with the processing periodicity of channel300. In this example, that factor is 11.5=2. The sampling frequency atthe input to the DAC for channel 300 is Fs=2nX. The DAC 260 convertschannel 300 from a digital to an analog signal which is output onsubscriber line 172. On the receive side ADC 278 samples at Fs=2mX. Thenin variable rate decimator 282 there are two decimation stages 444 and442. In the first 444 decimation by the inverse integer fraction ½ isfollowed by decimation by the factor m in stage 442. At the input to theDFT 204 the sampling frequency is Fs=X. Subsequently each coefficientset is gain scaled, decoded, tone reordered and deframed in module 222.

In FIG. 4B the processing of the second channel 310 with expanded tonespacing is shown. After framing, tone ordering, RS encoding and gainscaling in module 236 the coefficients for each symbol are converted inthe IDFT module 220 from the frequency to the time domain. Thisconversion occurs at a sampling frequency of Fs=2X. Next the digitaltime domain data for each symbol set is passed to the variable rateinterpolator 254 which in the embodiment shown is part of the AFE,though such need not be the case.

In the variable rate interpolator 254 there is logically only oneinterpolation stage 412. The interpolation that occurs in stage 412 iscommon to both the signal processing for channel 300 with standard tonespacing and channel 310 with expanded tone spacing. The samplingfrequency at the input to the DAC for channel 310 is Fs=2nX. The DAC 260converts channel 310 from a digital to an analog signal which is outputon subscriber line 172. On the receive side ADC 278 samples at Fs=2mX.Then in variable rate decimator 282 there is one decimation stage 442where decimation by a factor m occurs. At the input to the DFT 204 thesampling frequency is Fs=2X. This is the frequency at which each toneset of channel 310 is converted from the time to the frequency domain asa set of complex coefficients. Subsequently each coefficient set is gainscaled, decoded, tone reordered and deframed in module 222.

FIG. 5 shows the various DMT frame sizes associated with the logicalmodem shown in FIG. 2, including a new frame structure suitable for theexpanded tone spacing associated with short subscriber loops whichqualify for elevated data rates. Various standard DMT frames 502-510 areshown each varying in size depending on the DMT protocol and symbolsize. The frames 502-510 are those associated with currentimplementations of the VDSL DMT standard. All frames contain one DMTsymbol and are processed at the standard processing interval of 250microseconds per symbol per channel. An “X” is drawn through frame 510to reflect the fact that the DFT/IDFT does not need to support the fullrange of tone sets called for by the VDSL standard. Instead by reducingthe processing interval for short haul channels by an integer fraction,e.g. ½ as discussed above a DFT/IDFT engine with only 2048 tones canprocess a channel with the same fidelity as a DFT/IDFT with a 4096 pointsample/tone set. This greatly reduces the complexity of the logicalmodem. A new frame structure 512 is proposed to handle the channelswhich are processed at this expanded tone spacing. That frame includestwo symbols 514-516. In an alternate embodiment of the invention therewould be two discrete frames.

FIG. 6A shows possible session allocations for the multi-channel logicalmodem shown in FIG. 2. FIGS. 6B-D show the possible scheduling in theDSP for each of the channels of each of the three sessions shown in FIG.6A Scheduling in the DSP 124 on the upstream and downstream paths ishandled by controllers 206 and 234 respectively operating from sessionsetup tables 214 stored in memory 212 as shown in FIG. 2. Thesecontrollers operate as schedulers handling the interleaving (See FIGS.6A-D) and delivery of each packet to the discrete Fourier transform(DFT) engine 204.

In FIG. 6A the modem is shown to support either two channels withexpanded tone spacing (See FIG. 6B); or one expanded channel and twostandard channels (See FIG. 6C; or four standard channels (See FIG. 6D).In each of FIGS. 6B-D a processing interval for the DFT/IDFT is shown.The boundaries of the processing interval are referenced as 600-602.Within this interval from t_(n) to t_(N+4) there are subintervals duringwhich all active channels are processed. The total time elapsed duringthe processing interval from 600-602 equals the standard processinginterval of 250 microseconds called for by the X-DSL DMT standards.Possible channels allocations or interleavings are shown for each of thethree session loadings B, C, D shown in FIG. 6A are shown incorresponding FIGS. 6B, 6C and 6D respectively. A channel with expandedtone spacing has more than one symbol/tone set processed in the standardprocessing interval of 250 microseconds. In FIG. 6B both channels 1-2are accorded expanded tone sets and both are processed twice in thestandard processing interval. In FIG. 6C the expanded tone set channel 1is processed twice in the standard processing interval. The channels 3and 4 which are accorded standard tone sets are processed once in thestandard processing interval. In FIG. 6D no channel is accorded expandedtone sets and all are processed once in the standard processinginterval.

FIG. 7 is a process flow diagram of the setup phase of operation of themodem shown in FIG. 2. Processing begins at process 700 in which themodem is initialized. Control then passes to process 702 in which theDSLAM controller 110 (See FIG. 1) allocates another channel to the modemline card. Next in process 704 the X-DSL DMT protocol for the channel isidentified. Control then passes to decision process 706. In decisionprocess 706 a determination is made for both the CO and subscriber modemunits as to whether expanded tone spacing is supported. If expanded tonesets are supported control passes to process 708. In process 708 thespacing of the expanded tone set is established by setting theprocessing interval for each of the symbol/tone sets for that channel atan integer fraction or multiple of the standard processing interval of250 microseconds. Control then passes to process 710 in which thetraining for the channel is effected. Then in process 712 the channel ischaracterized for each tone in the tone set. Next in decision process714 a determination is made as to whether the subscriber loop whichcarries the channel qualifies for high data rates associated with anexpanded tone spacing. If not control passes to process 720. If thechannel's subscriber line loop qualifies as a short haul line/channelwhich supports the expanded tone spacing then control passes to process730 in which state information is exchanged between the CO andsubscriber modems.

If alternately, in decision process 706 a determination is made thatvariable tone spacing is not supported by either modem then controlpasses to process 720. In process 720 training is conducted at thestandard tone spacing of 4.3125 kHz. Then in process 722 the channel ischaracterized. Next in decision process 724 the loop is qualified forthe appropriate tone set and protocol. If the loop does not qualifycontrol returns to process 702. If the loop qualifies control passes toprocess 730 in which state information is exchanged between the CO andsubscriber modems.

Next in decision process 732 a determination is made as to whether thereis available processing bandwidth for the modem. If there is controlreturns to process 702 for the prospective addition of the next channelto the session. If the capacity of the modem is fully utilized thencontrol passes from decision process 732 to process 738 in whichrun-time communications on all channels is commenced.

In the above discussed system the processing interval in the DFT/IDFTfor channels communicated over short loops was an integer fraction ofthe processing interval for long loops. In an alternate embodiment ofthe invention there would more than two tone spacings with the standardtone spacing of 4.3125 kHz utilized on intermediate length loops. Inthis embodiment of the invention longer loops would accorded processingintervals in the DFT/IDFT which are integer multiples of the processinginterval for the intermediate loop lengths. In this embodiment as wellthe shorter loops would continue to be processed at processing intervalin the DFT/IDFT which are an integer fraction of the processing intervalfor the intermediate loops. In still another embodiment of the inventionthe processing intervals would be either fractional multiples orfractions of the default processing interval of 250 micro seconds.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

1. An X-DSL transceiver configured to couple to at least one subscriberline to communicate at least one multi-tone modulated communicationchannel thereon; and the X-DSL transceiver comprising: a digital signalprocessor (DSP) configured to couple to the at least one subscriber linefor multi-tone modulation and demodulation of the at least onecommunication channel thereon, and the DSP operative during a trainingphase of the at least one multi-tone modulated communication channel todetermine an available range of frequencies on the at least onesubscriber line and to expand or contract a tone spacing of each of afixed number “N” of tones responsive to the determination; and the DSPincluding: a Fourier transform module for transforming successive tonesets of the fixed number “N” of tones of the at least one multi-tonemodulated communication channel between a frequency domain and a timedomain, and the Fourier transform module responsive to the determinationto expand the tone spacing by decreasing a processing intervalassociated with the transformation of each successive tone set betweenthe time and frequency domains and further responsive to thedetermination to contract the tone spacing by increasing the processinginterval associated with the transformation of each successive tone setbetween the time and frequency domains; and at least one analog frontend (AFE) coupled between the at least one subscriber line and the DSPand the at least one AFE configured to perform analog-to-digital anddigital-to-analog conversion of the at least one multi-tone modulatedcommunication channel at rates compatible with the processing intervalof the Fourier transform module; whereby the range of frequenciesspanned on the subscriber line between a lowest frequency one and ahighest frequency one of the fixed number “N” of modulated tones of theat least one communication channel includes either a first range offrequencies associated with expanded spacing of each of the “N” tonesand generally associated with a short subscriber line or a second,relatively narrower, range of frequencies associated with contractedspacing of each of the “N” tones and generally associated with arelatively long subscriber line.
 2. The X-DSL transceiver of claim 1,wherein the at least one AFE further comprises: a digital-to-analogconverter (DAC) coupled to the at least one subscriber line andperforming a digital-to-analog conversion of the at least one multi-tonemodulated communication channel at a fixed sample rate; and aninterpolator coupling the DSP to the DAC, and the interpolatorconfigured to vary an amount of interpolation of the at least onemulti-tone modulated communication channel in direct correspondence witha duration of the corresponding processing interval selected by the DSP,whereby tone sets processed in the DSP at the relatively shorterduration processing interval will be subject to relatively smalleramounts of interpolation and vice versa, thereby allowing the DAC tomaintain the fixed sample rate.
 3. The X-DSL transceiver of claim 1,wherein the at least one AFE further comprises: an analog-to-digitalconverter (ADC) coupled to the at least one subscriber line andperforming an analog-to-digital conversion of the at least onemulti-tone modulated communication channel at a fixed sample rate; and adecimator coupling the ADC and to the DSP, and the decimator configuredto vary an amount of decimation of the at least one multi-tone modulatedcommunication channel in direct correspondence with a duration of thecorresponding processing interval selected by the DSP, whereby tone setsprocessed in the DSP at the relatively shorter duration processinginterval will be subject to relatively smaller amounts of decimation andvice versa, thereby allowing the ADC to maintain the fixed sample rate.4. The X-DSL transceiver of claim 1, wherein the at least one subscriberline comprises a plurality of subscriber lines and the at least onemulti-tone modulated communication channel comprises a plurality ofmulti-tone modulated communication channels each associated with acorresponding one of the plurality of subscriber lines; and furthercomprising: the DSP configured to independently select for each of theplurality of multi-tone modulated communication channels a correspondingprocessing interval which inversely corresponds with the available rangeof frequencies on the associated one of the plurality of subscriberlines.
 5. The X-DSL transceiver of claim 1, wherein the DSP supportsmodulation and demodulation of the at least one multi-tone modulatedcommunication channel in a plurality of multi-tone protocols.
 6. TheX-DSL transceiver of claim 1, wherein the at least one AFE furthercomprises: a digital-to-analog converter (DAC) coupled to the DSP andthe at least one subscriber line and the DAC performing adigital-to-analog conversion of the at least one multi-tone modulatedcommunication channel at a sample rate which corresponds inversely withrespect to the processing interval selected by the DSP; and ananalog-to-digital converter (ADC) coupled to the communication mediumand the DSP and the ADC performing an analog-to-digital conversion ofthe at least one multi-tone modulated communication channel at thesample rate which corresponds inversely with respect to the processinginterval selected by the DSP.
 7. The X-DSL transceiver of claim 1,wherein the at least one subscriber line comprises a plurality ofsubscriber lines and the at least one multi-tone modulated communicationchannel comprises a plurality of multi-tone modulated communicationchannels each associated with a corresponding one of the plurality ofsubscriber lines; and further comprising: the DSP configured toindependently select for each of the plurality of multi-tone modulatedcommunication channels a corresponding processing interval whichinversely corresponds with the available range of frequencies on theassociated one of the plurality of subscriber lines; and a schedulercoupled to the DSP to schedule processing therein of the plurality ofmulti-tone modulated communication channels based on criteria includingassociated processing intervals for each of the plurality of multi-tonemodulated communication channels.
 8. The method of claim 1, wherein theexpanded tone spacing of each of the modulated tones substantiallycorresponds to a 8.625 kHz and the contracted tone spacing of each ofthe modulated tones substantially corresponds to a 4.3125 kHz tonespacing.
 9. A method in an X-DSL transceiver for communicating at leastone multi-tone modulated communication channel across a subscriber line;and the method comprising the acts of: determining during a trainingphase of the at least one multi-tone modulated communication channel anavailable range of frequencies on the at least one subscriber line;selecting an expanded or contracted tone spacing for each successive setof a fixed number “N” tones associated with the modulation anddemodulation of the at least one multi-tone modulated communicationchannel responsive to the available range of frequencies determined inthe determining act; transforming successive tone sets of the fixednumber “N” tones of the at least one multi-tone modulated communicationchannel between a frequency domain and a time domain; responding to theselection of an expanded tone spacing in the selecting act by decreasinga processing interval associated with the transformation of eachsuccessive tone set between the time and frequency domains; andresponding to the selection of a contracted tone spacing in theselecting act by increasing a processing interval associated with thetransformation of each successive tone set between the time andfrequency domains; performing analog-to-digital and digital-to-analogconversion of the at least one multi-tone modulated communicationchannel at rates compatible with the processing interval established inthe corresponding one of the responding acts; whereby the range offrequencies spanned on the subscriber line between a lowest frequencyone and a highest frequency one of the fixed number “N” of modulatedtones of the at least one communication channel includes either a firstrange of frequencies associated with expanded spacing of each of the “N”tones and generally associated with a short subscriber line or a second,relatively narrower, range of frequencies associated with contractedspacing of each of the “N” tones and generally associated with arelatively long subscriber line.
 10. The method of claim 9, wherein theexpanded tone spacing of the first responding act substantiallycorresponds to a 8.625 kHz and the contracted tone spacing of the secondresponding act substantially corresponds to a 4.3125 kHz tone spacing.